1. Field of the Invention
The present invention is directed to a method for error detection in digital communication systems having a communication network with switching networks.
2. Description of the Prior Art and Related Material
Analog and digital data transmitted on lines and cables are exposed to disturbing influences, i.e. electromagnetic fields. These disturbing influences modify the information to be transmitted; for example, bits are often "turned around" during the transmission. Since international long distance connections are particularly susceptible to bit errors, international standardization authorities such as CCITT have defined minimum prescriptions for bit error rates. Optical transmission links are considerably less subject to external influences, thus yielding extremely high transmission reliability.
In general, every transmission link is composed of two parts. One part relates to the transmission of information via a physical path between two nodes. Optical communication cables are increasingly being used for this part. A further part relates to the through-connection of the information in the nodes.
The potential disturbing influences in the transmission oriented part have been decisively reduced by using optical transmission systems. In comparison, bit errors frequently occur in the switching networks of the communication system during the through-connection events.
Due to their crucial significance for the communication systems, the switching networks usually incorporate double or triple redundancy. Redundancy makes it possible to through-connect the information over a number of switching network levels when a malfunction appears on the primary switching network level. One way this can be done is by having one switching network level in a quiescent (standby) condition, whereas the other switching network level carries out the active through-connection of the information. Another way to accomplish this is to have both switching network levels actively through-connect the corresponding information.
Bit falsifications during the through-connection of the bit stream are an additional problem in the art. They are caused by the aging of individual components. Such errors are difficult to analyze and eliminate.
An example of error detection methods for recognizing and correcting bit errors is disclosed in German patent application P 41 28 412.7. The application discloses a method wherein, before the actual through-connect event in the switching network occurs, the information words are duplicated and an additional check information such as, for example, a parity bit is attached to every information word. An information word is usually composed of eight bits. Also a parity bit is generally understood to be the binary checksum over the individual bits of an information word. The information words of a channel supplemented by their check information are thereby transmitted between interface elements and switching network elements in a time-division multiplexing method. An uneven parity bit, meaning the inverted binary checksum of a information word, is preferably also attached. The duplicated information words are each conducted via a switching network level and are recombined after the through-connect event. Subsequently, a binary checksum of the through-connected information word is again calculated and compared to the co-transmitted parity bit. When the co-transmitted parity bit and the newly calculated parity bit deviate from one another, there is a transmission error. Additionally, after the through-connect event, the two information words allocated to one another are compared to one another bit-by-bit. The bit-by-bit comparison combination with an evaluation of the co-transmitted parity bit allows an unambiguous statement in most instances regarding whether that information was correctly through-connected. When a transmission error is identified, it is retained in a corresponding table memory via a statistical function. Conclusions about the condition of the corresponding switching network level can thus be made over defined time spans. These conclusions are of a purely statistical nature and, in particular, are utilized when the bit-by-bit comparison in combination with the evaluation of the parity bit does not supply unambiguous results. When this is the case, that switching network level that exhibited the lowest bit error rate in the past can preferably be employed for the through-connection of the bit streams.
The problem in this method, however, is that an incorrect through-connection through a switching network level causes a permanent difference of the respective information words in the bit-by-bit comparison, even through a correct transmission was identified on the basis of the co-transmitted parity bits in every switching network level. Since the statistical quality data were accessed in the proposed method, the table memory is falsified given an existing, permanent wrong through-connection.